1. Field of the Invention:
The present invention relates to a method for reinforcing a ceramic shaped body, particularly ceramics useful for structural materials.
2. Description of the Prior Art:
Silicon ceramics, such as silicon nitride, silicon carbide, SiAlON, etc., and zirconia ceramics or ceramics having a low thermal expansion, such as cordierite, etc. exhibit excellent heat resistance and the thermal shock resistance. Therefore, these ceramics have been utilized as structural materials, such as engine parts of gas turbine or internal combustion engines. But, these ceramics can be brittle if there are large defects in the surface or in an inner portion, whereby stress concentrates in or near these defects under normal operation and therefore, mechanical failure occurs therefrom thereby lowering the strength. Particularly, when ceramics are shaped by injection molding process, extrusion molding process and the like, the strength of the sintered bodies is liable to be noticeably lowered due to the unevenness of surfaces and, cavities or cracks formed on the surface of the shaped body.
In order to solve these problems, the following processes have been heretofore known. That is, a glaze is coated on the surface of the shaped body and the coated shaped body is fired to form a compressed layer on the surface of the sintered body; or the surface of the sintered body is coated with CVD process; or the sintered body is re-fired at a high temperature to round the crack tips. However, the process wherein the glaze is coated on the surface of the shaped body, has the defects that a glass layer having a low melting point which is different in the composition from the ceramic shaped body, is formed on the surface of the sintered body, so that such a shaped body cannot be used at a high temperature and further the difference of between the thermal expansion of the material of the shaped body and the glass layer is large and therefore when the coated body is exposed to the glass layer is readily separated from the shaped body. The CVD process is expensive and it is difficult to completely remove cracks and cracks still remain. Hence, the strength cannot be highly increased.